Amorphous solids are those which have no crystalline structure or long range order. Silicate glasses are a common example of an amorphous solid, but other materials can form amorphous structures as well. For example, certain metal alloys can solidify to an amorphous state upon rapid cooling from a melt. Such metal alloys are known as amorphous or glassy metals.
Although a number of amorphous metals are known, they have found only limited commercial usefulness other than as a magnetic material, discussed below. Amorphous materials are often thermally instable, i.e., they crystallize at relatively low temperatures. Some amorphous metals are particularly brittle, which limits their use in applications requiring a ductile material. Also, some glass-forming compositions can be made amorphous only with difficulty. The widest commercial use made of amorphous metals to date has been as a soft magnetic material in transformers. In particular, the alloy formed by rapid quenching of an Fe-Ni melt with additions of metalloids such as B or P has yielded an amorphous metal suitable for this purpose.
U.S. Pat. No. 4,116,682 discloses a class of amorphous metal alloys of a ternary composition rich in Fe, Ni, Co, Cr, or Mn, but differing from the Fe-Ni glasses of the previous paragraph in that there are at least two glass forming additives present, which are chosen from two different groups of elements, one of which comprises a larger atomic size element such as Zr or Y and the other of which comprises a smaller atomic size element such as B, with the elements from both groups acting synergistically to produce glasses low in total additive concentrations (under 25 atomic percent of total additive). Al-based, Al-rich ternary compositions which are glass forming and contain representatives of two different additive groups are also known.
Among noble metal element alloys, especially those based on Ag and Au, Au.sub.80 Si.sub.20 is known to be readily glass forming, but this glass is not thermally stable at room temperature. The known glassy metal Au.sub.80 Ge.sub.12 Si.sub.08 has a temperature of crystallization T.sub.c of 307K. Binary Au-lanthanide systems form more thermally stable glasses, but require high concentrations, approximately 40 percent, of the additive elements for glass formation. Ag is notorious for being a poorly glass forming element; the only known binary glass-forming, Ag-rich alloys contain the divalent lanthanides Eu or Yb as additives, containing less than 82 atomic percent Ag. (It should be noted that both Eu and Yb are expensive addition elements.) Thus, amorphous metal alloys comprising at least 70 percent of the noble metals Ag and Au do not form upon rapid quenching at the cooling rates of melt-spinning, discussed below, or are not long-term thermally stable at room temperatures, or contain expensive addition elements. For some of the other noble metals, glass forming compositions are known, but, except for Pd.sub.80 Si.sub.20, these are generally not rich in the noble metals.
In the metallic glasses in general, and in the noble metal glasses contemplated in this invention in particular, there are advantages to obtaining compositions rich in the majority metal component or components (in the case of mixtures, e.g., of noble metals), because in such compositions the desirable characteristics of the majority component or components (magnetic, corrosion resistance, ductility, color and visual appearance) are most likely to be retained in the glassy state.
Amorphous solids may be formed by several processes. For bulk materials, the most commercially useful processes use liquid quenching techniques, in which a melt of the alloy composition is cooled rapidly, generally at a rate greater than 10.sup.5 K/s, so that no crystalline structure forms. The melt spinning technique, a type of liquid quenching, directs a stream of melt toward a rapidly spinning wheel, upon which the melt solidifies and is formed into a ribbon. Other processes, which are unsuitable for bulk quantity production, include vapor deposition processes, such as thermal vapor deposition and sputtering. Last, electroless chemical deposition is suitable only for certain elements not containing the noble metals.
Thus, there is a need for formulating alloy compositions rich in the noble metals, especially Au and Ag, that could be readily glass forming upon quenching from the liquid at cooling rates obtainable by melt spinning.